Conventionally, a waste oil generated when using edible oil, lubricating oil, etc., have been generally treated by combustion, and it has not been very common to recycle the waste oil. On the other hand, it is pointed out that there is a possibility to produce dioxin by combusting substances contained in the waste oil and the like, and thus there exists the present situation that combustion of the waste oil cannot be conducted without careful consideration. Accordingly, recycling methods in place of the combustion have been discussed, and there have been proposed methods for removing contaminants from the waste oil, for example, by filtration, adsorption or semi-permeable membrane.
However, even with these methods, the waste oil cannot always be turned to a reusable condition. From another aspect, since the waste oil usually contains substances which are oxidized such as edible oil and the like, a method for performing reduction treatment of the waste oil to reclaim the oil with a condition before use has been thought to be effective. Thus recently, the reduction treatment of the waste oil has particularly attracted attention and has been continuously studied.
As a specific example of the reduction treatment for the waste oil, there has been proposed a method where a strongly alkaline reducing water prepared by electrolyzing a water containing salt is added to the waste oil and the like to reduce the waste oil to a natural glycerin through chemical reaction (Japanese Patent Laid-open Application Publication 2002-114992).
However, in the aforementioned technology, since the salt solution is immixed, the reducing water must be separated from the reclaimed waste oil again, so that the waste oil cannot be treated continuously.
On the other hand, in order to inhibit oxidation degradation of oil, an antioxidant is generally added. However, it is known that, as the oil is used, the function of the antioxidant is lowered and is lost at the end.
Further, in order to maintain the function of the antioxidant added to the oil, the additional antioxidant must be added, but since the amount to be added is limited, a method for maintaining the function of the antioxidant has been desired.